Tyre pressures

The news and your views about biking
Post Reply
Godzilla
Posts: 462
Joined: 11 Mar 2014, 21:51
Your Bike:
Has thanked: 6 times
Been thanked: 38 times

Tyre pressures

Post by Godzilla »

An interesting article copied verbatim from an old MCN.

It goes someway to explain why manufacturers suggest one tyre pressure when riding a bike solo, two up, with luggage, slow and fast.

Discuss


It's important to be pumped
Tyre pressures are a crucial factor in determining how your bike handles and how
quickly you wear out your (not exactly cheap) tyres.
There are lots of myths and misconceptions about what pressures you should run in
the wet, on track days or when you're loaded with luggage. Usually you'll find someone
propping up the bar who knows better than the manufacturers' recommendations. To
find out how close they are to being right we talked to a genuine expert - a man who
should know tyres if anyone does.

Leo Smith spent years as chief development tester at Avon tyres. He is now
motorcycle product manager. He said: "We probably get asked more about tyre
pressures than about any other aspect of a tyre”. There's so much bad information
kicking about that people can't separate the truth from fiction."

Smith says that is largely the fault of tyre companies themselves. Several years ago,
different tyre companies recommended different pressures for different tyres and
different bikes. But around 10 years ago, a decision was reached between the
companies to standardise pressures so that most bikes can run on the same no matter
what tyres they're on. That standard is 36psi at the front and 42psi at the rear.

There are some exceptions, like some 400cc grey imports which run 29psi at the front
and 36psi at the rear. Another notable exception is the Kawasaki ZX-12R - which is
meant to run 42 front and rear. But if you've got a modern, mainstream bike,
chances are you should be running the 36/42 standard.

That 42 figure in particular will have a lot of the gentlemen at the bar shaking their
heads. But it is not a figure chosen at random. Pressures determine how your tyres
deflect. The lower the pressure, the more the tyre will flex. That may make for a
comfortable ride when you're cruising in a straight line, but the tyre will flex too fast at
speed and make your bike unstable. The bike will feel vague going into turns and feel
like it's going to tip into the corner suddenly. This is because the tyre isn't "strong"
enough and it's literally buckling under you.

The bike will also feel wallowy through turns and it'll weave under acceleration.
Conversely, if you over-inflate a tyre, the flex will be slower but that will make your bike
more stable at high speeds. The ride comfort and the tyre's ability to absorb shocks will
be lost and your wrists and backside will take the brunt of it. The bike will feel so harsh
that many people will think they have a suspension problem.

Cornering won't feel as bad as when pressure is too low, but you will again lose feel
and feedback from the tyres. For example, if you ride over a stone, an over-inflated
tyre cannot absorb it and the tyre breaks contact with the road. Smith says the classic
myth about tyre pressures is that you deflate them for wet-weather riding. He says
most grip comes from the tyre's compound and the contact patch - and the shape of
the tyre where it contacts the road is everything.

Tread patterns stop water from building up under the tyres - which could cause a bike
to aquaplane. Smith says: "A good front tyre chucks enough water out of the way to
enable the rear to get the power down. If you reduce the tyre pressure, the tread
becomes compressed so it can't clear as much water." If anything, Smith recommends
you increase the rear tyre by 2-3psi in the wet but leave the front as it is.

Another widely held misconception is that the psi recommendations are the maximum
the tyre can take. They're not. The figure only tells at what pressures the tyres were
tested at for all-round use. You could actually safely inflate a type up to around 50psi if
you really wanted to, although it wouldn't do you much good.

But the biggest area for debate has to be track days. If you've ever been to one it's
almost certain someone has told you you'll be best off reducing your tyre pressures.
You get more grip that way, they tell you. Smith has radically different advice.
You should leave them alone, he says. "Racing tyres are of a totally different
construction and stiffness to road tyres so they need less pressure to maintain the
carcass shape. That's where the rumours and bad advice comes from. "If you drop the
psi in road tyres you will get more movement in the tread pattern. They will heat up too
much and that will eat into tyre wear. You'll almost certainly ruin a set in a day without
gaining any advantage in grip."

Smith says he's known people to drop their rear tyre to just 22psi when heading for the
track. His advice is to leave your tyres alone, saying a good tyre at standard pressures
will give more grip than you need on a track day because you almost certainly won't be
going as fast or for as long as racers. Track surfaces offer much better grip than the
road, too - another reason for leaving your tyre pressures the same for the ride to the
track as for the ride around it.

Many people also ask the experts at Avon if they should increase psi to take pillion
passengers. Again there's no need. The manufacturers' agreed pressures of 36/42
were arrived at after testing with pillions, luggage, cold tyres and every other
combination you could think of.

One of the few cases when Smith does recommend you change your pressures is
when your tyres wear. A worn tyre has lost a lot of its strength as the shape and
flexibility levels have changed. That means it will handle differently to a new tyre. Try
increasing the tyres by 2psi when you're down to around 40 per cent tread depth. It will
only make a marginal difference, but it should improve your bike's handling a bit.
You may not have to keep changing your tyre pressures, but you do have to maintain
them. Smith recommends that you check them once a week as an absolute minimum
but to be extra safe, you should really check them every day because a tyre can
change by as much as 3psi on its own just because of changes in the weather.

You should always measure your tyre pressures when they are cold. A few bikes are
now coming with tyre pressure gauges in their under-saddle tool kits. If you haven't got
one it's worth buying one. They only cost a few quid and take up about as much room
as a pen. Forecourt gauges are notoriously inaccurate.
User avatar
Cavetroll87
Posts: 2320
Joined: 11 Mar 2014, 21:40
Your Bike: 2014 Yamaha MT09
Location: Kent
Has thanked: 591 times
Been thanked: 532 times

Re: Tyre pressures

Post by Cavetroll87 »

Interesting read, personally I always though t of it as a confidence thing, I run my tyres a little lower than standard because for me they feel more stable and it gives me more confidence to push as I feel like I get more feedback but I know people who don't drop them coz they prefer the feel of standard pressure, imo I think its whatever your most comfortable with
Remember: If in Doubt use Full Throttle, It may not make the situation any better, But it will end the suspense...
User avatar
Bratty
Posts: 1399
Joined: 12 Mar 2014, 10:03
Your Bike: Honda CB1000r
Location: Great Barr Birmingham
Has thanked: 418 times
Been thanked: 151 times

Re: Tyre pressures

Post by Bratty »

Great read but I always follow the rule if 36/42
I get knocked down but i get up again.
User avatar
Blade
Posts: 18772
Joined: 14 Mar 2014, 18:43
Your Bike: Kawasaki ZX10R
Location: North West
Has thanked: 3134 times
Been thanked: 3767 times

Re: Tyre pressures

Post by Blade »

Well I must be wrong then as I run between 32/34 front and 36/38 on the rear depending on conditions when road riding and personally find it an improvement.
User avatar
duke63
Posts: 15493
Joined: 22 Oct 2013, 07:34
Your Bike: Ducati 748/853 & Triumph Street Triple 765RS
Location: Staffordshire
Has thanked: 4170 times
Been thanked: 4124 times

Re: Tyre pressures

Post by duke63 »

Its hard to find recommended pressures from manufacturers now. I cant find any on the websites.
User avatar
Monty
Posts: 6671
Joined: 12 Mar 2014, 07:59
Your Bike: 2010 ZX6R
Location: Buxton
Has thanked: 2092 times
Been thanked: 2412 times

Re: Tyre pressures

Post by Monty »

Interesting that he only talks about cold pressures
Monty™© MCMLXXII
User avatar
Kwacky
Posts: 38538
Joined: 21 Oct 2013, 21:52
Your Bike: Brutale 800RR, 1000SX Ninja
Location: Brum
Has thanked: 4314 times
Been thanked: 8352 times

Re: Tyre pressures

Post by Kwacky »

That's pretty consistent though, which is why it's a good reference point. Otherwise we'll be getting the thermometers out.
User avatar
Deegee
Posts: 4206
Joined: 02 Apr 2014, 11:20
Your Bike: Daytona 675 & Tiger 900
Location: Côte d'Essex
Has thanked: 1046 times
Been thanked: 967 times

Re: Tyre pressures

Post by Deegee »

Not really, the rise in tyre pressure is directly related to carcass/wheel temp obviously, if you get the tyre pressure as soon as you come in after a session you can monitor how much quicker you're getting by the rise in temp, I can't remember the exact figure but iirc you should limit the hot pressure increase to something like 4-5psi from cold that way the tyre doesn't get too hard and the contact patch get too small. A proper tyre guy would know the details verbatim.
User avatar
Kwacky
Posts: 38538
Joined: 21 Oct 2013, 21:52
Your Bike: Brutale 800RR, 1000SX Ninja
Location: Brum
Has thanked: 4314 times
Been thanked: 8352 times

Re: Tyre pressures

Post by Kwacky »

But that's on the track, not the road.
User avatar
Cavetroll87
Posts: 2320
Joined: 11 Mar 2014, 21:40
Your Bike: 2014 Yamaha MT09
Location: Kent
Has thanked: 591 times
Been thanked: 532 times

Re: Tyre pressures

Post by Cavetroll87 »

So after reading this the other day I decided I would try putting my pressures back up to 36/42 as thats what pirelli recommend for my tyres on my bike (can look on their website, pretty cool) so after riding into work today my thoughts are that turn in has improved but my god they make the ride horrible as they now make every bump feel like I hit a curb and very skittish, I could probably dial this out with lots of time softening up suspension but being honest They feel a million times better running at slightly lower psi.

They noticeably take longer to warm up with lower psi but once warm its just a nicer smoother ride, just my thoughts and feelings on it after a quick experiment so I think ill be dropping back down to 34/36 as that always felt best to me (maybe its just what your used to)
Remember: If in Doubt use Full Throttle, It may not make the situation any better, But it will end the suspense...
User avatar
Itchy
Posts: 2544
Joined: 12 Mar 2014, 00:17
Your Bike: Kawasaki Z1000
Location: A galaxy far, far away...
Has thanked: 756 times
Been thanked: 983 times

Re: Tyre pressures

Post by Itchy »

I've got Avons on, I was a little surprised when the guy put 36/42 in when he fitted them but seeing as I had new tyres and had the "new tyre is better than the old squared off" feeling, then that recommended tyre pressure felt fine to me.
User avatar
Deegee
Posts: 4206
Joined: 02 Apr 2014, 11:20
Your Bike: Daytona 675 & Tiger 900
Location: Côte d'Essex
Has thanked: 1046 times
Been thanked: 967 times

Re: Tyre pressures

Post by Deegee »

Kwacky wrote:But that's on the track, not the road.
Yes - I should have been clearer, I was talking specifically about track pressures as the interviewee was talking about track pressures for a couple of paragraphs and I disagreed with some of his reasoning . A slow rider with only a couple of trackdays experience could probably leave his tyres at the normal pressures and be fine, but if he gets quicker the scenario I described will occur.

The article was written in 2000 according to MCN, no idea when Mr Smith actually worked for Avon though, what we should remember is that tyres have changed radically in that time, and advice given in good faith then is not guaranteed to be accurate 14yrs later.

Sorry if it sounds like I'm having a pop at Leo Smith - I'm not, I just disagree with some of the stuff he said back then given my experience on and around the track in recent years. DG.
User avatar
duke63
Posts: 15493
Joined: 22 Oct 2013, 07:34
Your Bike: Ducati 748/853 & Triumph Street Triple 765RS
Location: Staffordshire
Has thanked: 4170 times
Been thanked: 4124 times

Re: Tyre pressures

Post by duke63 »

Tyre technology has changed dramatically in 14 years. I doubt that advice is as relevant now.

Dunlop make sports tyres designed to be specifically run at lower pressures now to increase the amount of rubber on the track/road.

The one piece of advice that has not changed though is that tyres should be run at recommended pressures in the wet as they are designed to work best at that pressure. If you run them lower the tread closes up and stops water being dispersed from the contact area.
User avatar
Kwacky
Posts: 38538
Joined: 21 Oct 2013, 21:52
Your Bike: Brutale 800RR, 1000SX Ninja
Location: Brum
Has thanked: 4314 times
Been thanked: 8352 times

Re: Tyre pressures

Post by Kwacky »

I didn't realise that's 2000 advice.
Godzilla
Posts: 462
Joined: 11 Mar 2014, 21:51
Your Bike:
Has thanked: 6 times
Been thanked: 38 times

Re: Tyre pressures

Post by Godzilla »

Its interesting, and I have spoken to the Metzeler guys in person, and they were adamant that the pressure to run their tyres at were 36/42. Thats both the tyre and bike manufacturer giving those pressure. There must be a reason. Although the article is a few years old if the tyre manufacturers still have that agreement it still makes the 36/42 pressures valid even though tyres have advanced if thats the pressures they are being designed to run at.

Thinking about it when they spend lots of money designing a tyre Im sure that pressures come into the process somewhere and 36/42 must be best as thats what they are designed and made to run at.
User avatar
D6
Posts: 2781
Joined: 14 Mar 2014, 18:26
Your Bike: Speed Triple 1050SE a bit DEAD + S3 RS
Location: Staffordshire
Has thanked: 212 times
Been thanked: 653 times

Re: Tyre pressures

Post by D6 »

I would imagine some will be simply a liability reason. It might not necessarily be the optimum pressure (36/42) in every situation, but will be safe in every situation. Therefore it is the value they give to offer safety advice. Running it at anything different is then at the owners risk.
Post Reply